The present invention relates to a vehicle seat belt retractor which has a tension reducing mechanism for reducing the tension in a seat belt that extends around a vehicle occupant.
One known seat belt retractor has two springs for biasing a seat belt storage spool to rotate in a belt retraction direction relative to a housing. The two springs are connected in series with a ratchet wheel interposed between the two springs. One end of the first spring is fixed relative to the housing, and the other end is attached to act on the ratchet wheel. One end of the second spring is attached to the ratchet wheel, and the other end is attached to act on the spool. Upon withdrawal of the stored seat belt from the spool, the ratchet wheel is rotated in a belt withdrawal direction, and the first and second springs are stressed.
The retractor has a mechanism which is actuatable to block rotation of the ratchet wheel relative to the frame in the belt retraction direction. Thus, when the mechanism is actuated, only the spring force of the second spring is applied to bias the spool. Thus, a reduced tension is placed upon the seat belt and the seat belt applies a reduced tension force to the vehicle occupant.
The first spring has a stronger spring force than the second spring. When the mechanism which blocks rotation of the ratchet wheel is deactuated, the ratchet wheel is permitted to rotate in the belt retraction direction, and the first spring drives the ratchet wheel in the belt retraction direction. The rotating ratchet wheel accelerates to a relatively high velocity and winds the second spring as a result of the larger spring force of the first spring being applied to the weaker second spring. If sufficient stress is present in the first spring, the second spring may be wound solid. Repeated application of the force of a highly stressed first spring on the second spring may cause unnecessary stress in the second spring.